Sex pheromones of insects are each a biologically active substance typically released from female individuals and having a function of attracting male individuals. They exhibit high attractive activity even in small amounts. Sex pheromones have been used widely as means for forecasting the emergence or confirming geographical distribution (invasion into a specific area) of insect pests, or means for controlling insect pests. The means for controlling insect pests includes mass trapping, lure & kill or attract & kill, lure & infect or attract & infect, and mating disruption, which have been widely provided in practical use. To utilize sex pheromones, production of a necessary amount of each of the synthetic sex pheromones at a low cost may be needed for basic research and moreover, for application.
Planococcus minor (common name: passionvine mealybug, which will hereinafter be abbreviated as “PVMB”) is also called Pacific mealybug or Guava mealybug. It is an economically important insect pest because it is widely distributed geographically in the Temperate and Tropical zones and causes damage to many crops. Ho et al. isolated the sex pheromone of this pest insect and determined that it is (E)-2-isopropyl-5-methyl-2,4-hexadienyl acetate (J. Chem. Ecol., 33, 1986-1996 (2007)).
The activity of a sex pheromone is largely influenced by not only the carbon skeleton but also the double bond position or geometry with respect to the structure thereof. In fact, it is reported in the above article by Ho et al. that with respect to the pheromone of PVMB, (Z)-2-isopropyl-5-methyl-2,4-hexadienyl acetate, which is the geometric isomer, inhibits the activity. For fundamental biological research or agricultural research and moreover, for providing for application or practical use, a sufficient amount of synthetic pheromone should be supplied. There is therefore an eager demand for the development of a highly efficient and highly selective production method in which an amount of isomeric byproduct is small so that no purification is required.
Ho et al. have reported the synthesis of the sex pheromone of PVMB. According to the reported synthesis, the intended product is obtained by subjecting ethyl 3-methyl-2-oxobutanoate to three steps, containing a Wittig reaction, a reduction reaction and an esterification reaction, respectively, to obtain a geometric isomer mixture of the intended product and (Z)-2-isopropyl-5-methyl-2,4-hexadienyl acetate, which is the geometric isomer of the intended product; and separating the intended product from the geometric isomer mixture through high-performance liquid chromatography (HPLC) (J. Chem. Ecol., 33, 1986-1996 (2007)).
Millar et al. have reported the synthesis of the sex pheromone of PVMB containing no geometric isomer by starting from a propargylic alcohol compound and using an addition reaction of isopropylmagnesium bromide to form a tri-substituted olefin as a key reaction (Tetrahedron Letters, 49, 315-317 (2008)).